DESCRIPTION

The kernel object system implements an object-oriented programming system
in the FreeBSD kernel. The system is based around the concepts of
interfaces, which are descriptions of sets of methods; classes, which are
lists of functions implementing certain methods from those interfaces;
and objects, which combine a class with a structure in memory.
Methods are called using a dynamic method dispatching algorithm which is
designed to allow new interfaces and classes to be introduced into the
system at runtime. The method dispatch algorithm is designed to be both
fast and robust and is only slightly more expensive than a direct
function call, making kernel objects suitable for performance-critical
algorithms.
Suitable uses for kernel objects are any algorithms which need some kind
of polymorphism (i.e., many different objects which can be treated in a
uniform way). The common behaviour of the objects is described by a
suitable interface and each different type of object is implemented by a
suitable class.
The simplest way to create a kernel object is to call kobj_create() with
a suitable class, malloc type and flags (see malloc(9) for a description
of the malloc type and flags). This will allocate memory for the object
based on the object size specified by the class and initialise it by
zeroing the memory and installing a pointer to the class’ method dispatch
table. Objects created in this way should be freed by calling
kobj_delete().
Clients which would like to manage the allocation of memory themselves
should call kobj_init() with a pointer to the memory for the object and
the class which implements it. It is also possible to use kobj_init() to
change the class for an object. This should be done with care as the
classes must agree on the layout of the object. The device framework
uses this feature to associate drivers with devices.
The functions kobj_class_compile(), kobj_class_compile_static() and
kobj_class_free() are used to process a class description to make method
dispatching efficient. A client should not normally need to call these
since a class will automatically be compiled the first time it is used.
If a class is to be used before malloc(9) is initialised, then
kobj_class_compile_static() should be called with the class and a pointer
to a statically allocated kobj_ops structure before the class is used to
initialise any objects.
To define a class, first define a simple array of kobj_method_t. Each
method which the class implements should be entered into the table using
the macro KOBJMETHOD() which takes the name of the method (including its
interface) and a pointer to a function which implements it. The table
should be terminated with two zeros. The macro DEFINE_CLASS() can then
be used to initialise a kobj_class_t structure. The size argument to
DEFINE_CLASS() specifies how much memory should be allocated for each
object.

HISTORY

Some of the concepts for this interface appeared in the device framework
used for the alpha port of FreeBSD 3.0 and more widely in FreeBSD 4.0.